The present invention relates to a process for continuous production of methyl mercaptan by reacting an educt gas mixture of methanol and hydrogen sulfide in the vapor phase at a reaction temperature between 300.degree. C. and 500.degree. C. and under an operating pressure of 5 to 15 bar over a catalyst bed containing aluminum oxide, with subsequent separation of the methyl mercaptan from the gas mixture by absorption and distillation, with recycling of the unconsumed methanol and hydrogen sulfide, as well as removal of inert gases and waste water and replacement of consumed methanol and hydrogen sulfide.
Methyl mercaptan is an important industrial intermediate for synthesis of methionine and for production of dimethyl sulfoxide and dimethyl sulfone. At present it is produced predominantly from methanol and hydrogen sulfide by reaction at an aluminum oxide catalyst. Methyl mercaptan is usually synthesized in the gas phase at temperatures between 300.degree. C. and 500.degree. C. and at pressures between 1 and 25 bar. The catalyst is usually doped with potassium tungstate as an activator to increase its activity and selectivity. The reaction of hydrogen sulfide and methanol to produce methyl mercaptan is an exothermic process which releases 28,500 kJ per kmol methanol reacted.
The product gas mixture resulting from this known process contains not only the methyl mercaptan and water which are formed but also the unreacted starting materials, methanol and hydrogen sulfide, the byproducts dimethyl sulfide and dimethyl ether, and small proportions of polysulfides. The product gas mixture also contains gases which are inert in this reaction: carbon dioxide, carbon monoxide, nitrogen and hydrogen.
The methyl mercaptan produced is separated from the product gas mixture in several distillation and washing columns at temperatures between 10.degree. C. and 140.degree. C., as shown in German Patent 17 68 826. Other product streams contain excess hydrogen sulfide, methanol, the inert gases, and wastewater. Methanol is used preferably as the washing liquid.
Excess hydrogen sulfide is returned to the pressurized reactor as "recycled gas". The recycled gas contains not only hydrogen sulfide but also methanol, methyl mercaptan, dimethyl sulfide, and other components of the product gas mixture. The proportion of these components in the recycled gas depends on the quality of the separation process. The unconsumed methanol is also recycled into the feed gas mixture. The recycled methanol, like the recycled gas, also contains other components. The hydrogen sulfide and methanol that are consumed are replaced by adding fresh materials.
The overall process of methyl mercaptan production can be divided into two parts. The first part includes pretreatment of the feed gas mixture and its conversion to methyl mercaptan. The second part includes the separation of the product gas mixture to recover methyl mercaptan, recycling of unconsumed methanol and hydrogen sulfide, and disposal of wastewater and waste gases. This invention is concerned with improvements in the first part of the production process.
The nature of the pretreatment of the feed gas mixture, its heating to the reaction temperature, and the subsequent cooling of the product gas mixture to condense and separate the methyl mercaptan have substantial effects on the economics of the overall process. High electrical power is required to operate the compressor, and large heating and cooling capacities are needed.
German Patent 17 68 826 has very little information about this first part of the production process. It can be seen from the process schematically presented that the recycled gas, together with the hydrogen sulfide make-up gas, is heated by the hot product gas mixture in a heat exchanger. The product gas mixture is cooled at the same time. The methanol required for the reaction is mixed into the hydrogen sulfide to produce the feed gas mixture after the hydrogen sulfide has been heated in the heat exchanger, shortly before entry into the reactor. The methanol washed from the product gas mixture is removed from the circulating wash liquid for that purpose. The amount removed from the circulating liquid is replaced with make-up methanol.
According to French Patent 2 477 538, make-up hydrogen sulfide gas for methyl mercaptan production is compressed in a compressor to 11 bar. Then recycle gas from the process, which contains hydrogen sulfide, dimethyl sulfide, methanol, and small proportions of methyl mercaptan is added to the compressed hydrogen sulfide to produce the feed gas mixture. The temperature of the feed gas mixture is raised to 510.degree. C. in a preheating furnace. circulating wash liquid, which contains methanol and dimethyl sulfide, is added to the feed gas mixture before it enters the first of up to ten reactors connected in cascade. That reduces the reactor inlet temperature to 450.degree. C. More methanol is injected into the gas stream, partly as liquid and partly as gas, before the second and subsequent reactors. All or part of the heat released in the reaction can be absorbed by the heat of vaporization needed for the methanol.
German Patent 11 34 368 describes use of a multitude reactor to produce methyl mercaptan. The multitude reactor comprises a cylindrical container in which the catalyst tubes are arranged parallel with each other. The catalyst tubes are welded to tube sheets at the top and bottom, as in multitude heat exchangers. The spaces between the tubes are filled with heat-transfer liquid. Each catalyst tube has, for example, a screen at its lower end which holds the particulate catalyst. The feed gas mixture flows upward through the reactor.
The catalyst comprises activated aluminum oxide in the form of spherical particles of mesh size 8 to 14. It is preferable for the catalyst to be diluted with inert material, such as silica or fused aluminum oxide, in the lower section of the catalyst tubes, so that the inert materials make up about 75% of the particles in the lower third of the tubes. The proportion of inert material decreases from this height up to the upper section of the tubes, so that there is pure catalyst in the upper part of the tubes. The gradually reduced dilution of the catalyst in the direction of flow gives a more even evolution of heat, which simplifies temperature control.
German Patent 11 34 368 uses a eutectic mixture of diphenyl ether and diphenyl as the heat-transfer liquid. This cooling liquid is vaporized by the heat of reaction, and is condensed into a coolant tank and returned to the reactor. According to German Patent 11 34 368 the feed gas mixture is heated by heat exchange with the hot product gas mixture and the hot coolant vapors.
An object of this invention is to improve the process for producing methyl mercaptan and an enhancement of the economics of the overall process, with respect to both the capital cost and the operating power costs, through improved pretreatment of the feed gas mixture and better utilization of the reaction heat.